Anorthosite in the Piedmont Prov- 
ince of Pennsylvania 


A DISSERTATION 


PRESENTED TO THE FACULTY OF BRYN Mawr COLLEGE IN PART 
FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE 
OF Doctor or PHILOSOPHY 


By 
ISABEL F. SmitH 


Bryn Mawr, PENNSYLVANIA 
1923 


OUTLINE 


Introductiony: 25 .cfys5 fess se ons oo ee Pee ee ee 
Scope of) paper io... uve. ess see wee ec eee Ce ee ee 
Summary of all: known anorthosites 4,2, 4 owiehe «soe ee ee 

Physiographic features of the Honeybrook Sere Se OAS Sets Sone 
Location and ‘area 2.6050... davies | Bee oe ts os oe ee eee 
opography and ‘drainage (ici scan ts oe eee ee es 

Geologic:and structural: relations 4 ecco ste eee os ee 
Geologic history) 2925 baa ee weld ee tae bok Claee nS aad eee 
Structure’and. metamorphism (27000050). > « sang oo eee 
Description and field relations of the anorthosite and associated 

TOK oes Seis suk nets, aia la ees GP AIRE San eterna eC Lah cei de 

Petrographic description. 4400. 0 Ay eee ea ia ot Rte eo ok eee 
Lypical anorthosite and tactes~ 40. aaa or a eee Foe 
Typical anorthosite—megascopic and microscopic description ; chem- 

iCal: ata Sey nha es ee ote eae te 3 es ORK Cea 
Border facies—megascopic and microscopic description ............ 
Summary of the petrographic study of the anorthosite ............ 
Quartz monzonite—megascopic and microscopic description; chem- 
ical data | 2's Pees cca ek besos bs eee See ner anes (eK ote 

Origin of; the anorthosite |i 25355 }>..2. 5, bee ee ee 
Current hypotheses concerning the origin of anorthosite ........... 
Bawen'sitheory 25. Sco oe < 5 cleanness nie eee cere 
Miller's: theory*i,@2% 2s. Biss os Menino. eee ok ee 
Application of these hypotheses to the Honeybrook anorthosite .. 

Summary and ‘conclusions 5... 5. ae eatso sw Mates cele oor oaks et eee 

Bibhography ic. oe ia wae wes oda a's hola oe ad ahi eee EC Ce 


oN 


er 


| INTRODUCTION 
Scope of Paper 

It is the purpose of this paper to report upon the detailed study 
of a small body of anorthosite in eastern Pennsylvania. This 
study has been undertaken with a view to the determination of the 
origin of the anorthosite. 

This rock is found near Honeybrook, Pennsylvania, which is 
located about forty miles northwest of Philadelphia. At the 
surface the rock appears as an elliptical area measuring approxi- 
mately twenty square miles (52 square kilometers). Compared 
with the great masses of anorthosite in eastern Canada and New 
York. State, each of which is measured in thousands of square 
miles, the occurrence in the Honeybrook quadrangle is of small 





$6 ear & 
BS KiiromereRrns 
PIEDMONT UPLANDS Gm TRIASSIC LOWLANDS GB ANORTHOSITE AREA 


FIGURE I 


Map of southeastern Pennsylvania showing location of Honeybrook 
quadrangle 


4 ANORTHOSITE IN THE 


areal significance. This anorthosite is, however, an extremely 
pure type with very clear-cut boundaries. A careful petrologic 
study of the rock, therefore, should furnish data bearing upon 
the important and as yet unsolved problem of the origin of 
anorthosite. 


Summary of All Known Anorthosites 

Of the geologists who have contributed to the study of anortho- 
sites either in North America or in other countries, some have 
merely recognized the rock and recorded its occurrence, others 
have given a petrographic description and have, in a very general 
way, suggested the origin, while only a few have offered a de- 
tailed theoretical discussion of the origin of anorthosite or of 
monomineralic rocks as a class. 

The two most important occurrences of anorthosite in North 
America are those in New York State and in eastern Canada 
which, because of their bearing on this paper, will be referred 
to in some detail later. A third North American occurrence of 
importance is that of the Lake Superior region where anorthosite 
outcrops at intervals along the Minnesota shore-line over a dis- 
tance of nearly fifty miles. The rock, which is considered a 
differentiation product of the great Duluth gabbro lopolith, has 
been studied in turn by Irving, Van Hise, Leith, Lawson, Elftman, 
N. H., H. V., and A. N. Winchell, and Grout.t In Wyoming ? 
anorthosite occurs as an intrusive, closely associated with gabbro. 
Lindgren and Ransome® record the occurrence of a dike of 
anorthosite in the Cripple Creek district, Colorado. One may 
infer from the brief description of this highly altered rock 
that it is by no means a pure anorthosite. Bowen makes 
reference to this reported dike in a foot-note in his “ Problem 
of the Anorthosites,” where he states that the rock contains biotite 


1 References to the works of these authors may be found listed alpha- 
betically by author in the bibliography at the end of the paper. 

2 Blackwelder, E., U. S. Geol. Survey Atlas, Laramie-Sherman Folio (No. 
1723), L010: 

3 Lindgren, W., and Ransome, F. L., Geology and gold deposits of the 
Cripple. Creek district, Colorado. U. S. Geol. Survey Prof. Paper 54, 
Pp. 55, 1900. 


PIEDMONT PROVINCE OF PENNSYLVANIA 5 


and quartz, and evidently approaches a granite in composition.* 
Lawson,® in his paper on the Minnesota anorthosites, mentions 
northern New Jersey among the localities where he has observed 
rocks “almost éxclusively composed of an allotriomorphic gran- 
ular aggregate of basic plagioclase,’ but no reference to anortho- 
sites can be found in the more recent reports of the New Jersey 
survey. It is possible that Lawson had reference to certain 
feldspathic facies of the quartz diorite or gabbro of the Highland 
region which are known, respectively, as the Losee and Pochuck 
gneisses. E. C. E. Lord’ describes “labradorfels’ from Burnt 
Head, Monhegan Island, Maine, in association with norite and 
gabbro-pyroxenite. The anorthosite and pyroxenite, which are 
extreme differentiates of a noritic magma, occur as “ dike-like 
masses (segregation veins)” ... having “an irregular lenticu- 
lar form without persistency in strike or dip. They can rarely 
be followed for more than twenty to thirty yards, and terminate 
generally in narrow veins and stringers while merging laterally 
‘into the surrounding rock without definite planes of contact.” 
Between these two extreme» differentiates, “bands of extremely 
coarse-grained norite occupy mineralogically and geologically 
an intermediate position.” *® Evidently these feldspathic segre- 
gations, although numerous, do not occur in large masses. They 
may represent original banding in the norite comparable to the 
banding in the gabbro of the Isle of Skye® or in the Duluth * 

4 Bowen, N. L., The problem of the anorthosites. Jour. Geology, vol. 25, 
p. 233, 1917. 

5 Lawson, A. C., The anorthosites of the Minnesota coast of Lake Su- 
perior, Minn. Geol. and Nat. Hist. Survey Bull. No. 8, p. 7, 1893. 

6 Bayley, W. S., Iron mines and mining in New Jersey. Final Rept. Ser. 
State Geologist, N. J., vol. 7, pp. I12I-122, IgIo. 

7Lord, E. C. E., Notes on the geology and petrography of Monhegan 
Island, Maine. Amer. Geol., vol. 20, PP. 329-347, 1900. 

8 Op. cit., pp. 338-339. 

®Geikie, Sir A., and Teall, J. J. H., On the banded structure of some 
Tertiary gabbros in the Isle of Skye. Quart. Journ. Geol. Soc. London, 
vol. 50, pp. 645-650, 1894. 

10 Grout, F. F., Internal structures of igneous rocks; their significance 
and origin; with special reference to the Duluth gabbro. Jour. Geology, 
vol. 26, pp. 439-458, 1918. ‘ 


6 ANORTHOSITE IN THE 


gabbro. Brief mention is made by Pardee* of a thoroughly 
crushed and sheared anorthosite associated with a strongly lam- 
inated granite gneiss in Shoshone County, Idaho. These rocks 
are considered very ancient pre-Cambrian intrusives. To the 
writer’s knowledge no other occurrences of anorthosite have been 
reported in North America. 

In Norway, great masses of anorthosite associated with gabbro 
intrusives have been described by Kolderup,” Vogt,1? Kjerulf,™ 
and Helland ; +° in Sweden anorthosite is described by Hogbom ; ** 
in Volhynia, Kiev, Podolie, and Kherson districts of southern 
Russia by Ossowski,** Schrauf,'* Tarrasenko,’® von Chrustschoff,”° 
and others. Rocks composed exclusively of feldspar are reported 
by Dupare and Pearce *! from the region of Mount Koswinsky 
in the northern Urals. In Egypt,?? anorthosite was found in place 
by Newbold, in a mountainous region east of the Nile where it 


11 Pardee, J. T., Geology and mineralization of the Upper St. Joe River 
basin, Idaho., U. S. Geol. Survey Bull. 470, pp. 45-46, 1911. 

12 Kolderup, C. F., Die Labradorfelse des westlichen Norwegens, Ber- 
gens Mus. Aarbog, No. 5, 1896, No. 7, 1808, No. 12, 1903. 

13 Vogt, J. H. L., Uber anchi-monomineralische und anchi-eutektische 
Eruptivgesteine. Vid. Selsk. Skr. I, No. 10, 1908. 

14 Kjerulf, Die Geologie des siidl. und mittleren Norwegens, 1857. 

15 Helland collaborated with Vogt. 

16 Hogbom, A. G., Geol. Foren. Stockholm Foérhand., Band 31, 1900. 

17 Ossowski, Sur les labradorites en WVolhynie. Comptes Rendus de 
"Academie des Arts et Sciences de Krakavie, Mai, 1870. 

18 Schrauf, Studien an der Mineral species Labradorit. Sitz. der K. 
Acad. Wiss., Wein, Band LX, abt. I, 1860. 

19 'Tarrasenko, Uber den Labradorfels von Kamenoi-Brod. Abh. Nat. 
Gesell. Kiew, 1886. 

20von Chrustschoff, Beitrage zur Petrographie Volhyniens und Russ- 


lands. Tschermak’s Min. pet. Mitt. Band 9, pp. 470~527, 1888. von 
Chrustschoff after a careful petrographic study of some of the so-called 
labradorite rocks of Volhynia, classes them under the name perthitophyre 
(p. 527). 

21 Duparc and Pearce, Recherches géologiques et pétrographiques sur 
’Oural du nord. Mem. Soc. de Physique et d’Hist. Nat. Genéve, vol. 34, 
1902. 

22 Adams, F. D., Summary of occurrences of anorthosite, in his Report 
on the geology of a part of the Laurentian area lying to the north of the 
Island of Montreal, Geol. Survey Canada, Ann. Rept., new ser., vol. 8, pt. 
J, p. 133, 1895. 


PIEDMONT PROVINCE OF PENNSYLVANIA 7 


is thought to bear the same relation to the surrounding rocks 
as does the anorthosite in Canada. This rock is described by 
Sir William Dawson in his “Notes on useful and ornamental 
stones of ancient Egypt.” ?* Lacroix describes briefly an occur- 
rence of anorthosite in the Massif central of Madagascar. 
Among the basic non-volcanic rocks is a gabbroic series in which 
is found “dans le Betsiriry (Telomito, a lest de Miandrivazo) 

. associée a des gabbros a facies pegmatique, une anorthosite 
a gros éléments, exclusivement formée de labrador. Dans la 
région d’Antanimaro (Androy), M. J. Giraud a constaté l’ex- 
istence d’anorthosites labradoriques, renfermant par place de 
grands cristaux d’hypersthéene et formant une sorte de pegmatite 
de norite. Ces types interessants semblent étre exceptionnels.” ** 
The chemical analysis of the anorthosite shows the rock to be very 
pure (1.5.4. 4-5. Labradorose.) Anorthosite has been re- 
ported from India.2> “ Labradorite rock” from the Lizard Pen- 
~ insula in Cornwall is an extremely localized facies in a gabbro 
area,?° 

Returning to the greatyoccurrences of anorthosite in North 
America, the anorthosite in eastern Canada covers a large part of 
the provinces of Quebec and Ontario, and is found also in New 
Brunswick and Manitoba; in the northern Adirondacks the 
anorthosite occurs largely in Clinton, Franklin, and Essex coun- 
ties. The Canadian anorthosite has been investigated pre-em- 
inently by Adams; that of the Adirondacks by Emmons, Kemp, 
Ruedemann, Cushing, Miller, and others, all of whom have writ- 
ten extensively on the pre-Cambrian intrusives. Recently, the 
theoretical discussion of the origin of anorthosite has been stimu- 
lated by Bowen’s paper, “The later stages in the evolution of 
igneous rocks,” 2" written in 1915, which was followed two years 

23 Dawson, Sir W., Trans. Victoria Inst. London, 1891. 


24 Lacroix, A., Les roches basiques non volcaniques de Madagascar. 
Extraits des Compt. Rend. des séances de l’Acad. des Sci., Inst. de France, 


t. 159, 1914. 

25 Brogeger, W. C., Die Eruptivgesteine des Kristianiagebietes, vol. 3, p. 
325, 1808. 

26 Information received verbally from Dr. T. G. Bonney, Cambridge, 
England. 


27 Jour. Geology, Supplement to vol. 23, No. 8, 1915. 


8 ANORTHOSITE 


later by “The problem of the anorthosites.” ®* In the latter 
paper Bowen supplies data from the Canadian and Adirondack 
anorthosites to support his theory. The paper gave rise to a 
succession of articles by Cushing 2° and Miller °° who took issue 
with Bowen on his interpretation of the Adirondack intrusives. 
Miller’s conclusions are best summed up in the paper entitled 
“ Adirondack anorthosite.” 84 The essential points of discussion 
between Bowen and Miller will be stated in a later section of this 
paper dealing with criteria bearing upon the question of the origin 
of the Pennsylvania occurrence. 

28 Tbid., vol. 25, pp. 200-243, 1917. 

29 Tbid., vol. 25, pp. 501-509, and 512-514, I917. 

80 Geol. Soc. America Bull., vol. 29, pp. 399-462, 1918; also Bull. N. Y. 
State Mus., Nos. 211, 212, 213, 214, 1919; also Jour. Geology, vol. 20, pp. 


20-47, 1921. 
31 Geol. Soc. America Bull., vol. 29, pp. 399-462, 1918. 


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ngle showing the bearing of the limits of the ano i 


rangement. 


Topographic map of a portion 


FIGURE 2. 


PHYSIOGRAPHIC FEATURES OF THE HONEYBROOK 
ANORTHOSITE 


Location and Area 


The Pennsylvania anorthosite is located in the eastern part of 
the state, in the Honeybrook quadrangle, Chester County. It 
lies between (40°00’) and (40°10’) north latitude and between 
(75° 45’) and (75° 55’) west longitude. The area of anorthosite 
measures approximately six miles in length and three and one- 
half miles in width. 


Topography and Drainage 

The Honeybrook quadrangle falls within that portion of the 
Appalachian Highlands which is known as the Piedmont province 
and which comprises the Piedmont uplands and the Triassic low- 
lands. The northern third of the Honeybrook quadrangle lies 
within the Triassic lowlands, the southern two-thirds within the 
Piedmont uplands. The Piedmont province is composed of a 
series of dissected plateaux extending as a broad upland to the 
southeast of the Appalachian Mountains. The summits of these 
plateaux are the vestiges of successive peneplains which have 
been traced ** throughout the Piedmont province. In the Honey- 
brook district the anorthosite area represents a remarkably well- 
defined, small, physiographic and lithologic unit. It is a dome- 
shaped upland, elliptical in outline, within which are found a 
few isolated remnants of the Honeybrook peneplain at altitudes 
of about 700 feet. This clear-cut oval area of the anorthosite 
is surrounded by quartz monzonite except on the south where 
a fault brings the Cambrian quartzite series against the intru- 
sives. . 

The configuration of the surface in Upper Cretaceous time, 
when regional uplift renewed erosion on the Honeybrook pene- 
plain, may be hypothetically reconstructed from the following 
interpretation of the present drainage systems. The East Branch 

32 Bascom, F., Cycles of erosion in the Piedmont province of Pennsyl- 
vania, Jour. Geology, vol. 29, pp. 540-559, 1921. 

10 


t 


PIEDMONT PROVINCE OF.PENNSYLVANIA 11 


of Brandywine Creek (see figure 2) is now flowing approximately 
parallel to and within a half-mile of the northern border of the 
anorthosite. In early youth, the course of the stream may have 
been controlled. by the line of least resistance, which was the 
northern contact between the anorthosite and the quartz mon- 
zonite; later, with the deepening of the channel, this line of 
contact continually shifted to the left (figure 3) parallel to the 
channel, and along the slope of the valley. At the present time, 


=e 
Se 


ica Fic. 4 
-Section at Barneston Section at Birdell 
I. Quartz monzonite 

2. Anorthosite 

3. Gambrian quartzite 

4. Baltimore gneiss 






— Honeybrook Level <— 


— Present Level << 





the stream possesses a deeper and wider valley. Hence, the 
quartz monzonite is even further removed laterally from the 
channel, while the parallelism between the line of contact and 
the direction of the stream’s course is still pronounced. 

Again, along the southwest boundary of the anorthosite, the 
present course of the West Branch of Brandywine Creek sug- 
gests a somewhat similar history. The stream is seen to be 
flowing along the contact between the quartz monzonite and the 
anorthosite. In its infancy, this stream was probably working 
on quartz monzonite which covered the anorthosite, and was 
making use of the fault-plane which furnished a well-defined 
line of least resistance. (Figure 4.) Later, by vertical cutting, 
it found the contact between quartz monzonite and anorthosite. 
Eventually, with further deepening of its channel and broaden- 
ing of its valley wall, the stream should lie well within the bound- 
ary of the anorthosite, the quartz monzonite should disappear on 
the south, while the fault-line should shift farther and farther 
away from the channel, parallel to the stream valley. 







12 ANORTHOSITE IN THE 


This initial economy of effort manifested in the early adjust- 
ment of these streams to their environment, has its analogue in 
the culture of the region. This may be seen on the topographic 
map which shows that the circumference of the ellipsoidal an- 
orthosite area, which is so clearly defined by the courses of the 
streams, is further accentuated by railroad lines and towns, all of 
which are confined to these stream valleys. The entire central 
area, higher and less accessible, is exclusively farmland. This 
upland interior is a water-shed from which small tributary 
streams radiate to the bordering main streams. At an altitude 
approximating 600 feet there is evidence of a break in the erosion 
cycle. Above this level the land rises gradually, while below, 
it dips steeply. This indicates accelerated stream action fol- 
lowing a period of retardation. The surface above the 600 foot 
contour interval may represent erosion during the Harrisburg 
cycle. This cycle was interrupted by uplift which renewed erosion 
and accelerated vertical cutting, producing the steeper slopes 
below the 600 foot contour. Erosion levels later than the Har- 
risburg are not recognizable in the area of the anorthosite where, 
with the exception just mentioned, no traces are preserved of 
the slight interruptions in erosion during late Tertiary and Pleis- 
tocene time. 


GEOLOGIC AND STRUCTURAL RELATIONS 


Geologic History 


That portion of eastern Pennsylvania which is called the Pied- 
mont province, is a part of the very ancient land mass called 
Appalachia. In pre-Cambrian time this land mass extended along 
the Atlantic border, its eastern limit reaching beyond the present 
shore line, its western border submerged beneath the Appalachian 
Sea. “ Appalachia contributed the materials and the Appalachian 
Sea was the basin in which the materials were laid down in beds 
that ultimately, after they had been folded, uplifted, and eroded, 
formed the Appalachian Highlands.” ** 

The oldest of the pre-Cambrian rocks represented within the 
Honeybrook quadrangle are the Baltimore and Pickering gneisses. 
Before the beginning of the Paleozoic era these sediments which 
were laid down in the shallow waters of the Appalachian Sea 
were first metamorphosedyduring earth movements which were 
accompanied by intrusions of igneous material. In the Honey- 
brook region the masses of quartz monzonite, anorthosite, gabbro, 
pyroxenite, and peridotite belong to this period of intrusion. 
That a prolonged period of erosion followed this uplift, bring- 
ing about the removal of much of the sedimentary covering and 
exposing portions of these intrusives even before the-close of 
pre-Cambrian time, is proved by the fact that in the Honeybrook 
area the basal formations of the Paleozoic series (Cambrian 
quartzite series) were laid down directly upon the crystalline 
floor. Deposition in Paleozoic time, in the Piedmont upland, 
was probably confined to the Cambrian and Ordovician periods 
during which arenaceous, arenaceous-argillaceous, calcareous, and 
argillaceous sediments were laid down along the borders of the 
Appalachian Sea. Of these sediments, which were closely folded 
by the end of the Paleozoic, only the arenaceous (Cambrian 
quartzite series) and the calcareous (Cambro-Ordovician lime- 
stone series) are found in the Honeybrook quadrangle. “ The 

33 Bascom, F., U. S. Geol. Survey Atlas, Elkton-Wilmington Folio (No. 
211), p. 15, 1920. 

13 


14 ANORTHOSITE IN THE 


uplifting of the sediments above the sea was probably not a con- 
tinuous process, but intermittent, and while erosion did not keep 
pace with the\upward movement, Paleozoic topography un- 
doubtedly never exhibited a constructional form, i.e., the arches 
and troughs of the folded crystallines never existed unmodified 
by erosion. Before the beginning of the next period of sedi- 
mentation of which there is a record (Triassic), they had been 
eroded to a relatively even surface.” ** 

Triassic sedimentation is well represented in this region. The 
Stockton, Lockatong, and Brunswick formations occupy ‘the 
northern third of the Honeybrook quadrangle (see frontispiece) 
where they were laid down successively in the shallow waters 
of the trough-like inland basin bordering the crystallines on the 
northwest. The trace of the surface of uncomformity at the 
base of the Triassic marks the line of division between the Tri- 
assic lowlands and the Piedmont uplands. Igneous action dur- 
ing the Triassic is represented by extrusions and intrusions of 
basalt and diabase as flows, sills, and dikes. Diabase occurs 
in the Honeybrook quadrangle as small, narrow, discontinuous 
dikes and larger irregular dike-like masses. The former are 
scattered throughout the quadrangle while the latter seem to be 
almost entirely confined to the Triassic formations. These igne- 
ous intrusions represent the youngest rocks in the region, From 
the close of the Triassic to the present time there has been no 
submergence of this district. Beginning with post-Triassic time, 
the river systems drained to the southeast into the Atlantic 
Ocean. The streams have in general retained their courses to the 
present day, and denudation of the region has been continuous 
except for minor pauses and accelerations. 


Structure and Metamorphism 


The igneous materials intruded during the pre-Cambrian earth 
movements which lifted and folded the Baltimore and Picker- 
ing sedimentary series accumulating upon western Appalachia, 
were themselves subjected later to pressure and metamorphism 
at three different times. At the close of the Ordovician period 


34 Bascom, F., U. S. Geol. Survey Atlas, Philadelphia Folio (No. 162), p. 
17, 1909. 


PIEDMONT PROVINCE OF PENNSYLVANIA 15 


both sedimentary and igneous rocks probably suffered uplift and 
slight folding. Again, from Middle Devonian through the 
Permian they were subjected to severe and more or less con- 
tinuous earth movements with close folding and faulting, which 
resulted in the recrystallization of the Cambrian, Cambro- 
Ordovician and Ordovician sediments into quartzites and quartz 
schists, calcareous schists and marbles, and mica schists, respec- 
tively, and brought about the conversion of the pre-Cambrian in- 
trusives into orthogneisses. Later, toward the close of the Tri- 
assic period, earth movements contemporaneous with the intrusion 
and extrusion of igneous material produced further metamorph- 
ism throughout the region. During the prolonged erosion interval 
from post-Triassic time to the present, thousands of feet of 
Paleozoic sediments have been removed. In the Honeybrook 
area, the great folds of the Cambrian quartzite series and Cambro- 
Ordovician limestone series appear for the most part only as 
narrow, alternating ridges and valleys which are the roots of 
closely compressed anticlines and synclines trending northeast 
and southwest. These longitudinal belts of the Paleozoics are 
separated from one another by broad intervening areas of pre- 
Cambrian sedimentary and igneous rocks. The anorthosite and 
associated rocks are found in about the centre of the northern 
area of pre-Cambrian intrusives. 


Description and Field Relations of the Anorthosite and Associated 
Rocks 


In the field the anorthosite is found in contact with quartz 
monzonite except where, along the southern margin, a fault has 
brought the Cambrian quartzite series against the anorthosite. 
The anorthosite may be readily distinguished from the surround- 
ing rocks because of its grayish-blue color and its characteristic 
weathering. The color is due to the development of a secondary 
mineral, zoisite, which has almost entirely replaced the original 
feldspathic constituent. The weathering is spheroidal; the fields 
are strewn with large, well-rounded, light grayish-blue, tough 
boulders which are smooth-surfaced owing to the uniformity 
of composition of the rock. The dark-colored, oxidized surfaces 
of the weathered quartz monzonite, on the other hand, are rough 


16 ANORTHOSITE 


and pitted, and the rock is easily crumbled and disintegrated 
owing to the weathering out of the less resistant mineral cen- 
stituents. The slopes of the quartzite ridge to the south of the 
anorthosite are covered with buff-colored, flat, prismoid rock 
fragments which are easily cleavable into thin slabs with shiny 
surfaces due to “small flakes of glistening sericite developed 
along the bedding planes.” ** Gabbro is found in scattered areas 
over the Honeybrook district, but it does not anywhere occur in 
contact with the anorthosite. Small narrow diabase dikes trend- 
ing northeast and southwest penetrate the anorthosite but cannot 
be traced for long distances. Pegmatite dikes intrude the an- 
-orthosite near the southern boundary. 

85 Bliss, E., and Jonas, A., Relation of the Wissahickon mica gneiss to 


the Shenandoah limestone and to the Octoraro mica schist of the Doe Run- 
Avondale district, Coatesville quadrangle, Pennsylvania, p. 21. 


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QUARTZ MONZONITE Rae 
ANORTHOSITE ES 


GEoLocic MAP oF THE ANORTHOSITE AREA 





PETROGRAPHIC DESCRIPTION 


Typical Anorthosite and Facies 


It has already been stated that the Honeybrook anorthosite is 
a remarkably uniform rock. The type, characteristic of the whole 
mass, is a medium- to coarse-grained rock composed exclusively 
or chiefly of labradorite or a closely related feldspar, which is 
strongly altered to zoisite. It is this alteration product which 
gives the rock its characteristic light bluish-gray color. Fresh 
anorthosite has been obtained from one or two localities only, 
and it is in these localities alone that the feldspars are clear and 
glassy, and easily determinable. Anorthosite which had been 
freshly blasted was found to be quite as thoroughly zoisitized 
as the weathered blue boulders so characteristic of the region as 
a whole. Four quarries, which had been opened in the anortho- 
site for pegmatitic or other intrusions, furnished no less zoisitized 
material. 

The only variation from pure anorthosite is found close to the 
boundaries of the area where the rock is streaked with narrow 
irregular bands of mafic constituents. The pure anorthosite 
and the border facies will be described in detail. 


Typical Anorthosite 


Megascopic description: In the hand specimen the fresh rock 
is medium- to coarse-grained, pinkish gray in color. It is com- 
posed almost entirely of clear, mutually interfering crystals of 
feldspar showing bright cleavage surfaces. The feldspars for the 
most part are of uniform size, not exceeding four or five milli- 
meters in diameter. There are, however, a few exceptional laths 
measuring over two centimeters in length. In the more highly 
altered specimens which contain zoisitized or sericitized areas in 
great amount, the rock becomes blue-gray in color, and the feld- 
spars are clouded. Ferromagnesian constituents in very minute 
quantity may be found in a few specimens where they form faint 
narrow bands. Iron oxide stains are sometimes visible. 

Microscopic description: In thin section the texture of the — 

18 - 


PIEDMONT PROVINCE OF PENNSYLVANIA 19 


rock is seen to be holocrystalline, medium- to coarse-grained, in- 
equigranular. From 95 to 100 per cent. of the rock is composed 
of feldspar which is for the most part labradorite. The only 
other primary constituents are the accessory minerals apatite and 
magnetite, both of which are found in very small amounts as 
scattered grains; and very scanty biotite. The following minerals 
occur as secondary products: zoisite, sericite, clinozoisite, epidote, 
green hornblende, calcite, and leucoxene. Usually in the pure 
type, zoisite and calcite are the only alteration products found. 
Rarely, actinolite and epidote occur in very small amount. The 
feldspars are the intermediate plagioclases, labradorite and andes- 
ine-labradorite, the former always predominating. In a specimen 
from Forrest, of which chemical analysis was made, the feldspar 
so far as determinate under the microscope proved to be andesine, 
but many of the feldspars were obscured by zoisitization. The 
chemical analysis indicates that labradorite predominates over 
andesine. The feldspar crystals vary greatly in size; crystal 
boundaries occur rarely, if ever. Zonal growth is absent. Albite 
twinning alone, or combingd with pericline or Carlsbad twinning, 
is common. Zoisite and sericite form minute aggregates along 
cleavage cracks, in irregular fractures, or within the feldspar 
crystals. Occasionally sericite may occur in larger flakes. Zois- 
ite shows the characteristic indigo blue or blue-gray, between 
crossed nicols. The colorless monoclinic epidote, clinozoisite, has 
slightly higher double refraction—first order yellow and red. 
Epidote, with its characteristic pistachio-green color, is un- 
doubtedly a decomposition product of the green hornblende. It 
is not found disseminated generally through the rock as is the 
zoisite, but confined to specific areas where, in association with 
actinolite, calcite, zoisite, and magnetite, it forms dense aggregates. 
The amphibole, which gives an extinction angle of 17 degrees 
with pleochroism greenish-yellow to yellowish-green, is actinolite. 
Although there is no indication of its being secondary to a py- 
roxene, the fact that diopside is found abundantly in the border 
facies suggests that pyroxene is in all cases the original constitu- 
ent. No titanite is found, but a few wedge-shaped crystals which 
are altered to zoisite probably represent this mineral. 

Chemical data: The results of the chemical analysis of a 


20 ANORTHOSITE IN THE 


specimen near Forrest are given below. The chemical analysis 
of typical Adirondack anorthosite from Mt. Marcy, will be given 


for comparison. 


Honeybrook Anorthosite 36 


Chemical analysis 


LO, nein aes mene ah etl e ats 52.86 
LO SOR teh rctrete. ste ee eee 26.68 
Bes: Sr Rate eer ee 1.03 
TOU) <5 aie sete oes ues sks Gin Runes 74 
MeO. reciente nL slates aietaes 38 
RAD ed Se ne Pee gc ee 10.93 
Ne O Bee el iets ola es no ae 4.44 
Ue Ses Sa eee oa ae ee .92 
8) Oras ia wring ance ee tas 1.49 
A rir eteneyl oo te tan eaters II 
AOS Pale dS SI. Gr Venton eet. Slt 25 
PaO a eure readers et ins ‘23 
IM Og aera tet Ie as ei a .02 
Sy iid ge ee Fee 05 
Ba Wawd Pee ek tee .03 
CO is Me Bea's wie ote als lott trace 
ZI OMe pigs FORESTS be trace 
100.26 


1.5."4.(4)5. grano-labradorose. 


Adirondack Anorthosite 37 


Chemical analysis 





DA Re cies arte al Ate eee 26.45 
FeO) WARM IES! TaN, 28 he mr ane 1.30 
Be) wii gnu a ase h ae Gls cease .67 
MRI Onee eae na ee Eee .69 
LEY GA A, Bit gate eee A SE Mk eh oc 10.80 
Ne OMe! te cies eee iim ae 4.37 
FOOD Fae ae winittee eats sta iene ae .Q2 
HAVO se is os aietaie ts «kDa ele 53 

100.20 





Norm 
Oulartz 3.2 oa tite es ree 54 
Orthoclases 7s aes as 5.56 
Albite tant ee we ees 37:73 
Anorthite:s :4020b. canes 50.04 
Dionside ik Swiss ge cee eae oe 1.94 
Hypersthene eto es sees 23 
Mapnetite sa.sm aici 3 ree eee 1.39 
Tinienste o.oo este ata ele ade .46 
SA DatEC eens tee ee .67 
| 4G Fa liad a bet ee Case Raid ts 5, 1.60 
Bad ise SR Es eee .10 
De ule eee aie bet eee — 
MnO) to as, She ates utes eee — 
100.26 

Norm 
Quarkti ce ee ee 1.92 
Orthotlase jr se-c4 tee ae ee 5.56 
Albite’@:. oS jee eee ee ee 37.20 
Anorthites Gaigeegece? eee 49.76 
Diopsidés.a ede ia de cheers eee 3.06 
Hypersthene 2 3255 0) eee .40 
Mauonetite” > s.22 oe one ee ene 1.86 
94 © pe eR odin ry IP 53 
_ 100.29 


1.5.4.(4)5. grano-labradorose. 


The Honeybrook anorthosite falls into Class I, order 5, rang 4 
(approaching rang 3) and subrang 5 (transitional to subrang 4) 


36 Anorthosite, one-half mile northwest of Forrest, Pa., W. T. Schaller, 


analyst. 


37 Anorthosite, summit of Mt. Marcy, Adirondack Mts., N. Y., A. R. 


Leeds, analyst. 


9 


PIEDMONT PROVINCE OF PENNSYLVANIA a 


of the quantitative system of classification, and as its mode is 
normative, the name becomes grano-labradorose. The Adiron- 
dack anorthosite falls into Class I, order 5, rang 4, and subrang 5 
(transitional to subrang 4). The name becomes grano-labra- 


dorose. 


Border Facies of the Anorthosite 


Megascopic description: This less purely feldspathic type of 
anorthosite is medium- to coarse-grained, light gray, streaked 
with irregular bands of mafic constituents. Occasional light 
green and gray areas indicate the presence of sericite and zoisite. 
In some localities the rock has a pseudo-porphyritic texture 


- which is produced by the wavy arrangement of the dark bands 


about what appear to be large single feldspar crystals. On close 
examination, however, the light areas are found to be composed 
not of single phenocrysts, but of many small interlocking crystals 


of feldspar. A few feldspar crystals measure from one to al- 


most three centimeters on their longer axis, but the average size 
of both the light and dark constituents is less than half a centi- 
meter. 

Microscopic description: The texture is subhedral, inequi- 
granular. The primary constituents are andesine-labradorite and 
diopside, with accessory apatite, magnetite, biotite, and rarely 
quartz. The secondary constituents are actinolite, sericite, epi- 
dote, zoisite, chlorite, and epidote. Garnets are comparatively 
rare. Much of the diopside is altered to actinolite, chlorite, and 
epidote. Some of the pyroxenes show beautiful reaction rims 
of epidote. The proportions of the constituents are feldspar 75 
to go per cent, mafic minerals 10 to 25 per cent. 


Summary of the Petrographic Study of the Anorthosite 

The anorthosite is a medium- to coarse-grained feldspathic rock 
composed almost entirely of interlocking crystals of labradorite. 
The crystals vary in size from a few millimeters to two or three 
centimeters across their greater diameter. Granulation of the 
feldspars, which is such a common feature of the Morin and 
Adirondack anorthosites, is not found in these rocks. The Lake 


22 ANORTHOSITE IN THE 


Superior anorthosite is reported** to be free from secondary 
textures. The only indications of strain subsequent upon con- 
solidation of the Honeybrook anorthosite are the universal twin- 
ning and the occasional distortion or bearing of the plagioclase 
crystals. There is no perceptible difference between the grain 
at the centre of the area and that at the borders. The medium- 
to coarse-grained character persists to the limits of the anortho- 
site mass. From the comparatively coarse-grained, light bluish- 
gray hornblendic anorthosite along the periphery which contains 
andesine-labradorite feldspars, one passes rather abruptly to the 
finer-grained, dark-colored quartz monzonite with its indis- 
criminate assemblage of dark and light constituents containing 
feldspars much higher in potash and soda, and also containing 
quartz. 

Except along the south where Cambrian quartzite is faulted 
against the intrusives, the anorthosite is everywhere surrounded 
by quartz monzonite. The quartzite will not be described since 
it has no bearing upon the origin of the anorthosite. Scattered 
areas of gabbro, and a few small dikes of metapyroxenite and 
metaperidotite are found in the Honeybrook quadrangle, but not 
in contact with the anorthosite. 

Quartz Monzonite 

Megascopic description: This is a medium- to fine-grained, 
medium-dark gray rock composed of quartz, feldspar, pyroxene, 
amphibole, and biotite. The rock shows in some places a foliated 
texture. 

Microscopic description: The texture is holocrystalline, inequi- 
granular. The feldspars are orthoclase, microperthite, micro- 
cline, albite, oligoclase, and andesine, all of which are partially 
altered to kaolin, sericite, albite, and zoisite. Quartz is present 
in variable amounts. The pyroxene is augite, or less commonly 
diopside; it may be altered to hornblende, chlorite, or epidote. 
Biotite occurs scantily but may be entirely absent. Accessory 
constituents are apatite, zircon, ilmenite, magnetite, and, more 
rarely, garnet, pyrite, and rutile. The percentages of the mineral 
constituents vary within a wide range. The average proportions 

88 Winchell, A. N., Mineralogical and petrographic study of the gabbroid 


rocks of Minnesota, and more particularly of the plagioclasytes. Amer. 
Geol., vol. 26, p. 216, 1900. 


oes 


PIEDMONT PROVINCE OF PENNSYLVANIA 23 


are approximately quartz 15 per cent., feldspar 55 per cent., mafic 


constituents 30 per cent. 


Chemical data: The analysis of the quartz monzonite is given 


below. 


Chemical Analysis 39 


SL a a 64.64 
AUCs shes WAGES Aig AR 15.92 
PR) Piste nivale Ose d hls > © 1.14 
Pe Mine ei Veh cate Sata tesla, 4.65 
Teg NC OC BG ena Ae een a 23 
SLO SI OR Uh aie le ORD ei Zi2 
Isms cae GR hae ea i 4.38 
DEL mR AE NIMS wee eee ND 6.06 
eet hard e. 4 anes ee so 04 
ELAM RSH oR of AR enema ae 43 
eee ha Sia anda 'e 3 6 42 
ZrO, La Ea ie al don ME trace 
PO: AORN ITT WER. tecut hls 5 '. trace 
SRE asain it. irk hi ohh eee ols .06 
ESL, fA oMRR RE SRA RES le ee .03 
AST) Ss budtne!s Rigs eeae Pane perees .10 

[00.22 


Norm 
CUBETe Rees ee Tsk pence) 8.70 
Clrthoclasesee aes Li ieiae hens 36.14 
Albee hip akin. vs otter wee 37.20 
PADONEIteRi tii ie te en ake 5.50 
PODSIIE beat ne dt A le ttt ee 4.40 
Eiypersthene.'. waste sacar. ete 5.15 
Marnetitene. verse esct eo eee 1.62 
Himenite sian orca hata  etite .76 
EVES ais cath ot ateceenct eeietin te xe 12 
EL ts ok ais sno nissan a 47 
Mri eatice © Sirens ergs 03 
Bary tat ce Rec reeraueee 10 


In the quantitive system of classification the symbol of this 
rock becomes I (II).(4)5.(1)2.3., and its name grano-pulaskose. 


89 Quartz monzonite, south of Ludwig’s Corner, Phoenixville quadrangle, 


Pa., R. C. Wells, analyst. 


ORIGIN OF THE ANORTHOSITE 


Current Hypotheses Concerning the Origin of Anorthosite 


Certain prevailing characteristics of anorthosite must be taken 
into account in any theory of the origin of the rock. These may 
be enumerated briefly. Anorthosite occurs characteristically as 
a deep-seated rock which is notably coarse or coarse- to medium- 
grained. Protoclastic texture and granulation are common phe- 


nomena. The rock has no effusive equivalent. Whether or not 


it has intrusive (dike) equivalents is still an open question. 
Bowen maintains that for the formation of small dikes anorthosite 
should contain 15 to 20 per cent. of metasilicates or other con- 
stituents such as orthoclase and quartz.*® Miller, however, finds 
that in the Lake Placid region of the Adirondack Mountains, 
anorthosite which is much poorer in metasilicates occurs com- 
monly in the overlying Grenville series.** Adams ‘*? does not 
record the occurrence of anorthosite in the form of small dikes 
in the Canadian areas. It would doubtless be conceded that 
anorthosite containing more than 95 per cent. feldspar would 
not flow and would not appear as dikes. 

Although it is generally believed that anorthosite is a dif- 
ferentiate of gabbroic magma, few petrologists have described 
the process by which this differentiation is effected. The for- 
mation of the Lake Superior anorthosite by accumulation of 
crystals of feldspar was suggested in 1893 by Elftman.4* In 
1900, N. H. Winchell, in describing the same region, speaks of 
the feldspar crystals as separating from the gabbroic magma ac- 
cording to their specific gravity.** 

Comparatively recently Bowen, in the Geophysical Laboratory 
in Washington, has made a study of the rock by means of ex- 

0 The problem of the anorthosites. Jour. Geology, vol. 25, p. 242, 1917. 

41 Adirondack anorthosite. Geol. Soc. America, Bull., vol 209, p. 426, 1918. 

42 Geol. Soc. America, Bull., vol. 28, p. 155, 1917. 

43 22nd Ann. Rept. Minn. Geol. and Nat. Hist. Survey, p. 178, 1894. 

44 Final Report, Minn. Geol. and Nat. Hist. Survey, vol. 5, pp. 66-67, 
1898-1900. 

24 


> 


| : 
eae 


PIEDMONT PROVINCE OF PENNSYLVANIA 25 


perimental work with artificial melts, especially of the ternary 
system diopside-anorthite-albite, and has compared his con- 
clusions *° with those drawn from field observation. The direct 
application of accurate experimental data to the problems of 
petrogenesis involved in monomineralic rocks is of great impor- 
tance and interest. Previous to Bowen’s articles, certain pecu- 
liarities of anorthosite were not altogether appreciated or per- 
fectly understood. 


Bowen’s Theory 

Bowen’s chief contribution to the question of the origin of an-- 
orthosite is the conclusion that anorthosite as such cannot exist 
in the liquid phase.** Only by the separation and sorting of 
plagioclase crystals from a magma which is probably gabbroid, 
can anorthosite be formed. The process by which this sorting 


_ occurs may be stated briefly as follows: One must presuppose 


a magmatic chamber of sufficient size and at sufficient depth to 
allow for the retention of heat within the enclosed magma over 
a long period of time. Asethe magma approaches the crystalliza- 
tion temperature, the mafic minerals are the first to separate out. 
Almost simultaneously with the metasilicates, calcic plagioclase 
of composition approximating bytownite (Ab,An,) begins to 
crystallize. While the mafic crystals, because of their high 
specific gravity, will sink almost as soon as formed, the calcic 
plagioclase will float in the magma because its density more 
nearly approaches that of the liquid. Experiment shows‘? that 
Ab,An, has a density of 2.733; while that of diabase glass at 
room temperature is 2.763. Heat will cause the liquid to expand 
more than the crystals, the difference of expansion up to 1200° 
being about 2 per cent. This nearly neutralizes the above rela- 
tion. The presence of volatile constituents in natural magmas, 
as opposed to dry melts, tends to make the crystals very slightly 
heavier than the liquid. While the crystals are suspended in the 
liquid they gradually change in composition from Ab,An, to- 
ward Ab,An,. The loss of the metasilicates has produced an 


45 The problem of the anorthosites. Jour. Geology, vol. 25, 1917. 
pe Citurp, 211. 
47 Bowen: private communication. 


26 ANORTHOSITE IN THE 


impoverished residual liquid which is increasingly more silicic, 
and in which the content of mineralizers present in the original 
magma becomes more and more concentrated., The plagioclases 
will be enriched in soda just so long as the density of the liquid 
will support them. After they have reached the composition 
Ab,An, they are no longer in equilibrium with the liquid; hence 
they will sink and gradually form a closely packed mass of plagio- 
clase of approximately labradoritic composition. The lighter 
residual magma which is squeezed out is enriched in albite, ortho- 
clase, and quartz, and upon crystallization forms further silicic 
‘differentiates, or solidifies as one rock type, according to whether 
the residual magma is cooled slowly or rapidly. Bowen interprets 
the syenite associated with the Adirondack anorthosite as a 
residual differentiate of the gabbroid magma. Any later dif- 
ferentiate may have intrusive relations in respect to an earlier 
one. Hence the intrusion of the syenite into the anorthosite 
is not inconsistent with this theory. 

It is evident that if anorthosite is formed by such a process 
as this, it should be essentially solid, as Bowen maintains, and 
devoid of interstitial liquid. We should therefore expect to find 
that any movement subsequent to the settling of the feldspar 
crystals would produce protoclastic texture or granulation. This 
may be a common feature in anorthosite.*® 


Miller's Theory 


Bowen’s theory has not met with entire favor among the 
Adirondack geologists. As before stated, the anorthosite of the 
northern Adirondacks is intimately associated with syenite which 
Bowen wishes to interpret as a salic, and therefore later, differ- 
entiate of the same magma from which the anorthosite and under- 
lying pyroxenite differentiated. Cushing *® admitted that Bowen’s 
theory was the most probable that had been brought forward. He 
was willing to concede that the syenite in the immediate area of the 
anorthosite might be so considered. Miller, on the other hand, 
insists that the anorthosite is older than the syenite, and he offers 
proof of their being separated by a gabbro which represents the 

48 The problem of the anorthosites. Jour. Geology, vol. 25, p. 218, 1917. 


#9 Structure of the anorthosite body in the Adirondacks. Jour. Geology, 
vol. 25, p. 50I, 1917. 


¥ 


ED 


PIEDMONT PROVINCE OF PENNSYLVANIA 21 


chilled border of the chamber in which the anorthosite was 
formed. The anorthosite was formed by the crystallization of 
the upper or residual portion of an intruded gabbroic magma 
from which many of the mafic constituents had settled out. The 
anorthosite as such was to some extent actually molten. Miller 
considers the anorthosite to be the only residual magma after the 
metasilicates have sunk to the bottom of the intrusive chamber. 
After approximately 9o per cent. of the latter had collected at 
the bottom, the magma above was a “ residual” anorthosite which 
was “to a very considerable degree at least, actually molten.” °° 

It must, however, be borne in mind that neither artificial melts 
nor natural systems have proved that metasilicates crystallize 
appreciably earlier than plagioclase; as before stated, their periods 
of crystallization are very nearly contemporaneous. The former, 
when once crystallized, will assuredly outstrip the latter in rate 


of sinking because of their greater density. Yet, unless it is 


supposed that the liquid contains something more than potential 
plagioclase, could it be maintained in a sufficiently fluid condition 
to permit the mafic crystal$‘to sink through the crystallizing feld- 
spars? Furthermore, without the presence of mineralizers to 
keep the liquid buoyed up and active, could the calcic plagioclase 
crystals change from Ab,An, to Ab,An, without developing 
zonal growth? In accordance with the “general law of increas- 
ing acidity in residual magmas,” as argued by Lane,*! we should 
expect to find, if not a felsic differentiate above the anorthosite, 
at least traces of a residuum either as interstitial material in the 
anorthosite, or in pegmatite dikes, intruding the chilled border. 
Miller makes no reference to pegmatites ; and while it is admitted 
that typical anorthosite is by no means a pure plagioclase rock, 
and that sodic and potassic feldspars and even quartz are some- 
times found in small amounts, the chemical analysis of typical 
Adirondack anorthosite indicates the presence of only .92 per 
cent. K,O, and there is not even a trace of the volatile constitu- 
ents common to nearly all igneous rocks. 


50 Bull. Geol. Soc. America, vol. 29, p. 461, 1918. 
51 Segregation granites. Jour. Geology, vol. 30, p. 163, 1922. 


28 . ANORTHOSITE IN THE 


Application of these Hypotheses to the Honeybrook Anorthosite 


The criteria which characterize Bowen’s and Miller’s theories, 
respectively, may be given in outline. 

Bowen’s theory carries with it the following criteria: 

1. Anorthosite should be a coarse-grained labradorite rock of 
very nearly monomineralic composition by reason of its being a 
collection of plagioclase crystals which have accumulated through 
gravitative settling. 

2. For the same reasons, anorthosite could never have been 
molten and should not, therefore, occur as dikes. The presence 
of protoclastic texture or granulation should be common to all 
moved anorthosites. 

3. Anorthosite should be associated with gabbro, pyroxenite, 
and peridotite, and with syenite or other silicic rocks. 

4. The silicic differentiate should overly the anorthosite ; it may 
or may not show intrusive relations with the anorthosite; there 
may be a transition facies between the two rocks. 

5. The gabbro, which represents the undifferentiated parent 
magma, should form a chilled border facies around and above 
the differentiated series. 

Miller’s theory is based on the following criteria in the Adiron- 
dack anorthosite: | 

1. Anorthosite is a coarse-grained labradorite rock of very 
nearly monomineralic composition by reason of its being the 
upper differentiate of a gabbroid magma from which the metasil- 
icates and orthosilicates have separated by gravitative settling.** 

2. A gabbroic chilled border surrounding and overlying the 
anorthosite marks the limits of the undifferentiated intrusion. 

3. Anorthosite may occur as dikes since it was once to some 
extent molten. Granulation, which is a common feature of an- 
orthosites, is caused by movement of the magma just prior to its 
solidification. 

4. The silicic rock, syenite, which partly overlies and surrounds 
and intrudes the anorthosite, is separated from the latter by the 


52 It seems to the writer, as already pointed out, that the conditions above 
outlined could not produce a rock which is either notably coarse-grained 
or so nearly monomineralic as the Mt. Marcy anorthosite which Miller calls 
typical anorthosite. 


Pee 


a 


PIEDMONT PROVINCE OF PENNSYLVANIA 29 


gabbroic facies. It therefore belongs to a distinctly later period 
of intrusion. 

The Honeybrook anorthosite region furnishes the following 
criteria : | 

1. The anorthosite is a coarse-grained, extremely pure mono- 
mineralic rock of the composition of labradorite. 

2. There is a conspicuous absence of anorthosite as dikes and 
outliers. 

3. There is no protoclastic texture. 

4. The border facies, because of its coarse grain, cannot repre- 
sent the chilled border of the parent magma. 

5. There is no intrusive relation between the anorthosite and the 
overlying quartz monzonite. 


SUMMARY AND CONCLUSIONS 


It is evident that the criteria of the Honeybrook anorthosite lend 
support to Bowen’s theory. 

The Honeybrook anorthosite forms a relatively flat, slightly 
domed, nearly horizontal mass which owes its origin to the sort- 
ing and accumulation of plagioclase crystals of the composition 
of labradorite (Ab,An,), and was therefore never fluid as such. 
The coarse-grained border facies is very like the anorthosite in 
composition, rarely more silicic. The change from anorthosite 
to the overlying quartz monzonite is abrupt, but the absence of 
intrusive relations between the two preclude the possibility of 
there being an appreciable difference in time between their periods 
of consolidation. The finer grain of the quartz monzonite is the 
result of the crystallization of an increasingly chilled residual 
magma. The anorthosite, on the other hand, owes its relatively 
coarse grain to the fact that in the early stages of crystallization 
when monomineralic rocks are produced, there is opportunity for 
the constant enlargement of the growing crystals. A few rem- 
nants of the quartz monzonite found on high land in the centre of 
the anorthosite area suggest that erosion has exposed only a por- 
tion of the anorthosite mass, all of which was once overlain by 
quartz monzonite. 

The pre-Cambrian igneous complex of the region around 
Honeybrook includes pyroxenites, peridotites, gabbros, quartz 
gabbros, anorthosites, diorites, quartz monzonites, and granites, 
all of which represent a series of differentiation products from 
a single great magma. Whether this complex represents batho- 
lithic or laccolithic intrusion cannot be ascertained from the 
structure of the pre-Cambrian gneisses and schists which once 
covered the region. 

In certain centres of this great intrusive chamber, conditions 
were evidently adapted to extreme differentiation. or the set- 
tling out of crystals, such as is implied in the formation of mono- 
mineralic rocks, it is necessary to assume that, locally at least, a 

30 


a) 


PIEDMONT PROVINCE OF PENNSYLVANIA 31 


tongue of dioritic or gabbroic magma forced a passage through 
consolidated material and spread sufficiently both laterally and ver- 
tically to form a chamber of considerable size in which heat was 
maintained for a long time. In this centre of differentiation the 
anorthosite and underlying pyroxenite settled with rather com- 
plete sorting, and the squeezed-up, residual magma solidified into 
quartz monzonite. Whether or not there was a still more silicic 
residue above the quartz monzonite cannot be determined, for 
much of the roof of the chamber was removed by erosion even 
before Paleozoic time. 


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HN 


VITA 


I, Isabel F. Smith, was born in Greeley, Colorado, on November 
15, 1890. My parents are Frederic Elias Smith and Margaret 
Emerson Smith. I received my early education in the public 
schools of Greeley, Colorado, and prepared for college at the 
Polytechnic High School in Los Angeles, California, and at the 
Misses Kirk’s School in Bryn Mawr, Pennsylvania. In 1911 I 
entered Bryn Mawr College and in 1915 received my A.B. degree 
in the group of Geology and Chemistry. In 1919 I received my 
M.A. degree. i Shay 

My graduate work was begun in 1917. The years 1917-1920, 
and 1921-1922 I studied in Bryn Mawr College; 1920-1921 I 
studied in Paris, France. In 1917-1918 and again in I919—1920 
I held a graduate scholarship at Bryn Mawr; in 1918-1919 I was 
a resident fellow. In 1918 I was awarded the President’s Euro- 
pean Fellowship. 

During the years 1915-1917 I was a teacher in the Mary C. 
Wheeler School, Providence, Rhode Island. 

At Bryn Mawr my graduate work in Geology was under 
the direction of Professor F. Bascom, Dr. Frank J. Wright, and 
Dr. Malcolm H. Bissell; in Chemistry under Professor James L. 
Crenshaw. | 

In Paris, I studied under Professor A. Lacroix in the Labora- 
tory of Mineralogy at the Muséum d’Histoire Naturelle. 

My preliminary examinations for the degree of Doctor of Phil- 
osophy were taken in January, 1922. My major subject was 
- Petrology, my associate minor was Organic Geology and Physi- 
ography, and my independent minor Inorganic Chemistry. 

The field work for my dissertation was undertaken during the | 
spring and summer of 1920. 

A paper entitled “ A Columbite Crystal from Boothwyn, Penn- 
sylvania,” appeared in the American Mineralogist, in October, 
IQI9Q. 

I wish to express my gratitude to Professor Bascom for her 
valuable aid and criticism in the preparation of my thesis; and for 
the interest shown and the inspiration given me during my years 
of study at Bryn Mawr. I wish also to express my gratitude to 
Dr. Bissel and Professor Crenshaw. 

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